1. Technical Field
The present invention relates generally to structural sandwich panels, and more specifically to multi-layered, oriented unidirectional fiber panels wherein the relative thickness of upper and lower laminae are manipulated to improve the load carrying properties of such panels without incurring a weight penalty.
2. Background
Various types of sandwich panels are disclosed in U.S. Pat. Nos. 3,963,846; 4,251,579; 4,292,356; 4,416,349; 4,990,391; and 5,106,668. Some panels have solid cores, while others are cavitated. A level, or smooth, surface covering is usually placed over the core material. Most commercial paneling is flat, although beneficial curved panels are known. (See U.S. Pat. Nos. 5,064,493 and 5,126,183).
Though there are many uses for structural sandwich panels, they have been found to be particularly useful in aircraft, as typically they are light but strong. U.S. Pat. Nos. 4,292,356; 4,416,349; and 4,990,391 disclose sandwich panels for aircraft noise attenuation purposes. This type of panel is meant to reduce interior noise by damping the vibrations of a reinforced skin structure. This is thought also to improve the sonic fatigue life of the vibrating structure and attached equipment.
U.S. Pat. No. 4,25 1,579 concerns the use of honeycomb sandwich panels in aircraft and other vehicles to fight fire. A fire-extinguishing fluid is disposed within the honeycomb cells and is held therein by facing sheets. The panel is arranged so that it is more rigidly enclosed on one side than the other. On impact, or entry of a projectile, the facing sheet on the less rigidly enclosed side of the panel preferentially breaks up or peels away so that the fire-extinguishing fluid is discharged in the preferred direction.
Honeycomb sandwich panels have long been used in aircraft flooring. Yet there remains a significant trade-off between strength and weight that is critical to aircraft design and operation. Aircraft flooring must have compressive properties sufficient to withstand the weight of loaded fixtures (such as seats), the walking of passengers and crew and the like, while at the same time being light enough so as not to create an undue weight burden. The advantages of stronger and lighter flooring materials being known, the search has continued for better designs.
U.S. Pat. No. 5,106,668 embodies one attempt to create a stronger structural sandwich panel. It uses a multi-layer dual honeycomb structure. A high-density first honeycomb core is attached to a second, low-density honeycomb core. The two honeycomb cores share a common septum or interior structural skin.
It is believed that unbalanced panels, such as an aluminum skin bonded to a balsa core, have been used in some aircraft flooring applications. Unbalanced woven glass skin panels are known to be used in bulkheads, cabinetry and smaller aircraft flooring.
It is an object of the present invention to advance the art of structural sandwich panels for use in aircraft flooring (and other applications) by further increasing the compressive strength and structural integrity of sandwich panels without incurring a weight penalty.